In electronic, microwave oriented navigation systems, such as a TACAN navigation system, the bearing from the TACAN navigation unit to a cooperating ground beacon is determined from the position-going-zero-crossings of the amplitude modulation components that are contained in the beacon pulses. In the operation of such systems, during a given time period there are usually a given number of positive-going-zero-crossings. The bearings are computed by the navigation system from the relationship of the multiple positive-going-zero-crossings that is the closest to the time period positive-zero-crossing.
In conventional TACAN navigational sets, the desired zero-crossings are determined using analog or hybrid analog/digital phase locked loop circuits. These systems, however, often suffer from problems of instability and drift commonly associated with analog phase locked loop components. Thus it is advantageous to have a new and improved digital phase locked loop circuit that uses an all digital approach and that substantially eliminates the problems of instability and drift and is capable of providing improved sensitivity through adaptive bandwidth switching and that reduces cost as the digital processing circuitry used can also be time shared to perform other necessary functions in the navigation system.